// Copyright (C) 2017 The Qt Company Ltd. // SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only /*! \page qtquick-modelviewsdata-modelview.html \title Models and Views in Qt Quick \brief how to display and format data in Qt Quick Most applications need to format data and display the data. Qt Quick has the notion of \e models, \e views, and \e delegates to display data. They modularize the visualization of data in order to give the developer or designer control over the different aspects of the data. A developer can swap a list view with a grid view with little changes to the data. Similarly, encapsulating an instance of the data in a delegate allows the developer to dictate how to present or handle the data. \image modelview-overview.png \list \li \b Model - contains the data and its structure. There are several QML types for creating models. \li \b View - a container that displays the data. The view might display the data in a list or a grid. \li \b Delegate - dictates how the data should appear in the view. The delegate takes each unit of data in the model and encapsulates it. The data is accessible through the delegate. The delegate can also write data back into editable models (e.g. in a TextField's onAccepted Handler). \endlist To visualize data, bind the view's \c model property to a model and the \c delegate property to a component or another compatible type. \section1 Displaying Data with Views Views are containers for collections of items. They are feature-rich and can be customizable to meet style or behavior requirements. \target qtquick-views A set of standard views are provided in the basic set of Qt Quick graphical types: \list \li \l{ListView} - arranges items in a horizontal or vertical list \li \l{GridView} - arranges items in a grid within the available space \li \l{PathView} - arranges items on a path \li \l{TableView} - arranges data from a \l QAbstractTableModel in a table \li \l{TreeView} - arranges data from a \l QAbstractItemModel in a tree \endlist These types have properties and behaviors exclusive to each type. Visit their respective documentation for more information. In addition, \l{Qt Quick Controls} contains some extra views and delegates that are styled according to the application style, for example \l HorizontalHeaderView and \l VerticalHeaderView. \section2 Decorating Views Views allow visual customization through \e decoration properties such as the \c header, \c footer, and \c section properties. By binding an object, usually another visual object, to these properties, the views are decoratable. A footer may include a \l Rectangle type showing borders or a header that displays a logo on top of the list. Suppose that a specific club wants to decorate its members list with its brand colors. A member list is in a \c model and the \c delegate will display the model's content. \snippet qml/listview-decorations.qml model \snippet qml/listview-decorations.qml delegate The club may decorate the members list by binding visual objects to the \c header and \c footer properties. The visual object may be defined inline, in another file, or in a \l {Component} type. \snippet qml/listview-decorations.qml decorations \image listview-decorations.png \section2 Mouse and Touch Handling The views handle dragging and flicking of their content, however they do not handle touch interaction with the individual delegates. In order for the delegates to react to touch input, e.g. to set the \c currentIndex, a MouseArea with the appropriate touch handling logic must be provided by the delegate. Note that if \c highlightRangeMode is set to \c StrictlyEnforceRange the currentIndex will be affected by dragging/flicking the view, since the view will always ensure that the \c currentIndex is within the highlight range specified. \section2 ListView Sections \l {ListView} contents may be grouped into \e sections, where related list items are labeled according to their sections. Further, the sections may be decorated with \l{qml-view-delegate}{delegates}. A list may contain a list indicating people's names and the team on which team the person belongs. \snippet qml/listview-sections.qml model \snippet qml/listview-sections.qml delegate The ListView type has the \c section \l{qtqml-syntax-objectattributes.html#Attached-properties-and-attached-signal-handlers} {attached property} that can combine adjacent and related types into a section. The \c section.property determines which list type property to use as sections. The \c section.criteria can dictate how the section names are displayed and the \c section.delegate is similar to the views' \l {qml-view-delegate}{delegate} property. \snippet qml/listview-sections.qml section \image listview-section.png \target qml-view-delegate \section1 View Delegates Views need a \e delegate to visually represent an item in a list. A view will visualize each item list according to the template defined by the delegate. Items in a model are accessible through the \c index property as well as the item's properties. \snippet qml/listview.qml delegate \image listview-setup.png \section2 Positioning of View Delegates The type of view will determine how the items are positioned. \l {ListView} will position the items in a straight line, depending on the \l {ListView::}{orientation}, while a \l {GridView} can lay them out in a 2 dimentional grid. It's \b {not} recommended to bind directly on \l {Item::x}{x} and \l {Item::y}{y}, since the view's layouting behavior will always take precedence over any positional binding. \section2 Accessing Views and Models from Delegates The list view to which the delegate is bound is accessible from the delegate through the \c{ListView.view} property. Likewise, the GridView \c{GridView.view} is available to delegates. The corresponding model and its properties, therefore, are available through \c{ListView.view.model}. In addition, any defined signals or methods in the model are also accessible. This mechanism is useful when you want to use the same delegate for a number of views, for example, but you want decorations or other features to be different for each view, and you would like these different settings to be properties of each of the views. Similarly, it might be of interest to access or show some properties of the model. In the following example, the delegate shows the property \e{language} of the model, and the color of one of the fields depends on the property \e{fruit_color} of the view. \snippet qml/models/views-models-delegates.qml rectangle \target qml-data-models \section1 Models Data is provided to the delegate via named data roles which the delegate may bind to. Here is a ListModel with two roles, \e type and \e age, and a ListView with a delegate that binds to these roles to display their values: \snippet qml/qml-data-models/listmodel-listview-required.qml document In most cases you should use \l{Required Properties}{required properties} to pass model data into your delegates. If a delegate contains required properties, the QML engine will check if the name of a required property matches that of a model role. If so, that property will be bound to the corresponding value from the model. In rare corner cases, you may want to transfer the model properties through the QML context rather than as required properties. If no required properties are present in your delegate, the named roles are provided as context properties: \snippet qml/qml-data-models/listmodel-listview.qml document Context properties are invisible to tooling and prevent the \l{Qt Quick Compiler} from optimizing your code. They make it harder to reason about the specific data your delegate expects. There is no way to explicitly populate the QML context from QML. If your component expects data to be passed via the QML context, you can only use it in places where the right context is made available via native means. This can be your own C++ code or the specific implementations of surrounding elements. Conversely, required properties can be set in a number of ways from QML or via native means. Therefore, passing data via the QML context reduces the re-usability of your components. If there is a naming clash between the model's properties and the delegate's properties, the roles can be accessed with the qualified \e model name instead. For example, if a \l Text type had (non-required) \e type or \e age properties, the text in the above example would display those property values instead of the \e type and \e age values from the model item. In this case, the properties could have been referenced as \c model.type and \c model.age instead to ensure the delegate displays the property values from the model item. For this to work, you need to require a \c model property in your delegate (unless you are using context properties). A special \e index role containing the index of the item in the model is also available to the delegate. Note this index is set to -1 if the item is removed from the model. If you bind to the index role, be sure that the logic accounts for the possibility of index being -1, i.e. that the item is no longer valid. (Usually the item will shortly be destroyed, but it is possible to delay delegate destruction in some views via a \c delayRemove attached property.) Remember that you can use integers or arrays as model: \qml Repeater { model: 5 Text { required property int modelData text: modelData } } \endqml \qml Repeater { model: ["one", "two", "three"] Text { required property string modelData text: modelData } } \endqml Such models provide a singular, anonymous piece of data to each instance of the delegate. Accessing this piece of data is the primary reason to use \e modelData, but other models also provide \e modelData. The object provided via the \e model role has a property with an empty name. This anonymous property holds the \e modelData. Furthermore, the object provided via the \e model role has another property called \e modelData. This property is deprecated and also holds the \e modelData. In addition to the \e model role, a \e modelData role is provided. The \e modelData role holds the same data as the \e modelData property and the anonymous property of the object provided via the \e model role. The differences between the \e model role and the various means to access \e modelData are as follows: \list \li Models that do not have named roles (such as integers or an array of strings) have their data provided via the \e modelData role. The \e modelData role does not necessarily contain an object in this case. In the case of an integer model it would contain an integer (the index of the current model item). In the case of an array of strings it would contain a string. The \e model role still contains an object, but without any properties for named roles. \e model still contains its usual \e modelData and anonymous properties, though. \li If the model has only one named role, the \e modelData role contains the same data as the named role. It is not necessarily an object and it does not contain the named role as a named property the way it usually would. The \e model role still contains an object with the named role as property, and the \e modelData and anonymous properties in this case. \li For models with multiple roles, the \e modelData role is only provided as a required property, not as a context property. This is due to backwards compatibility with older versions of Qt. \endlist The anonymous property on \e model allows you to cleanly write delegates that receive both their model data and the role name they should react to as properties from the outside. You can provide a model without or with only one named role, and an empty string as role. Then, a binding that simply accesses \c{model[role]} will do what you expect. You don't have to add special code for this case. \note The \e model, \e index, and \e modelData roles are not accessible if the delegate contains required properties, unless it has also required properties with matching names. QML provides several types of data models among the built-in set of QML types. In addition, models can be created with Qt C++ and then made available to \l{QQmlEngine} for use by QML components. For information about creating these models, visit the \l{Using C++ Models with Qt Quick Views} and \l{qtqml-typesystem-topic.html#qml-object-types} {creating QML types} articles. Positioning of items from a model can be achieved using a \l{Repeater}. \section2 List Model ListModel is a simple hierarchy of types specified in QML. The available roles are specified by the \l ListElement properties. \snippet qml/qml-data-models/listelements.qml model The above model has two roles, \e name and \e cost. These can be bound to by a ListView delegate, for example: \snippet qml/qml-data-models/listelements.qml view ListModel provides methods to manipulate the ListModel directly via JavaScript. In this case, the first item inserted determines the roles available to any views that are using the model. For example, if an empty ListModel is created and populated via JavaScript, the roles provided by the first insertion are the only roles that will be shown in the view: \snippet qml/qml-data-models/dynamic-listmodel.qml model \dots \snippet qml/qml-data-models/dynamic-listmodel.qml mouse area When the MouseArea is clicked, \c fruitModel will have two roles, \e cost and \e name. Even if subsequent roles are added, only the first two will be handled by views using the model. To reset the roles available in the model, call ListModel::clear(). \section2 XML Model XmlListModel allows construction of a model from an XML data source. The roles are specified via the \l [QML]{XmlListModelRole} type. The type needs to be imported. \code import QtQml.XmlListModel \endcode The following model has three roles, \e title, \e link and \e pubDate: \qml XmlListModel { id: feedModel source: "http://rss.news.yahoo.com/rss/oceania" query: "/rss/channel/item" XmlListModelRole { name: "title"; elementName: "title" } XmlListModelRole { name: "link"; elementName: "link" } XmlListModelRole { name: "pubDate"; elementName: "pubDate" } } \endqml The \c query property specifies that the XmlListModel generates a model item for each \c in the XML document. The \l{Qt Quick Demo - RSS News}{RSS News demo} shows how XmlListModel can be used to display an RSS feed. \section2 Object Model ObjectModel contains the visual items to be used in a view. When an ObjectModel is used in a view, the view does not require a delegate because the ObjectModel already contains the visual delegate (items). The example below places three colored rectangles in a ListView. \code import QtQuick 2.0 import QtQml.Models 2.1 Rectangle { ObjectModel { id: itemModel Rectangle { height: 30; width: 80; color: "red" } Rectangle { height: 30; width: 80; color: "green" } Rectangle { height: 30; width: 80; color: "blue" } } ListView { anchors.fill: parent model: itemModel } } \endcode \section2 Integers as Models An integer can be used as a model that contains a certain number of types. In this case, the model does not have any data roles. The following example creates a ListView with five elements: \qml Item { width: 200; height: 250 Component { id: itemDelegate Text { required property int index text: "I am item number: " + index } } ListView { anchors.fill: parent model: 5 delegate: itemDelegate } } \endqml \note The limit on the number of items in an integer model is 100,000,000. \section2 Object Instances as Models An object instance can be used to specify a model with a single object type. The properties of the object are provided as roles. The example below creates a list with one item, showing the color of the \e myText text. Note the use of the fully qualified \e model.color property to avoid clashing with \e color property of the Text type in the delegate. \qml Rectangle { width: 200; height: 250 Text { id: myText text: "Hello" color: "#dd44ee" } Component { id: myDelegate Text { required property var model text: model.color } } ListView { anchors.fill: parent anchors.topMargin: 30 model: myText delegate: myDelegate } } \endqml \target qml-c++-models \section2 C++ Data Models Models can be defined in C++ and then made available to QML. This mechanism is useful for exposing existing C++ data models or otherwise complex datasets to QML. For information, visit the \l{Using C++ Models with Qt Quick Views} article. \section2 Array models You can use JavaScript arrays and various kinds of QML lists as models. The elements of the list will be made available as model and modelData by the rules outlined above: Singular data like integers or strings are made available as singular modelData. Structured data like JavaScript objects or QObjects are made available as structured model and modelData. The individual model roles are also made available if you request them as required properties. Since we cannot know in advance what objects will appear in an array, any required property in a delegate will be populated, possibly with a coercion of \c undefined to the required type. The individual model roles are not made available via the QML context, though. They would shadow all other context properties. \section1 Repeaters \div {class="float-right"} \inlineimage repeater-index.png \enddiv Repeaters create items from a template for use with positioners, using data from a model. Combining repeaters and positioners is an easy way to lay out lots of items. A \l Repeater item is placed inside a positioner, and generates items that the enclosing positioner arranges. Each Repeater creates a number of items by combining each element of data from a model, specified using the \l{Repeater::model}{model} property, with the template item, defined as a child item within the Repeater. The total number of items is determined by the amount of data in the model. The following example shows a repeater used with a Grid item to arrange a set of Rectangle items. The Repeater item creates a series of 24 rectangles for the Grid item to position in a 5 by 5 arrangement. \snippet qml/repeaters/repeater-grid-index.qml document The number of items created by a Repeater is held by its \l{Repeater::}{count} property. It is not possible to set this property to determine the number of items to be created. Instead, as in the above example, we use an integer as the model. For more details, see the \l{qtquick-modelviewsdata-modelview.html#integers-as-models}{QML Data Models} document. If the model is a string list, the delegate is also exposed to the usual read-only \c modelData property that holds the string. For example: \table \row \li \snippet qml/repeaters/repeater.qml modeldata \li \image repeater-modeldata.png \endtable It is also possible to use a delegate as the template for the items created by a Repeater. This is specified using the \l{Repeater::}{delegate} property. \section1 Changing Model Data To change model data, you can assign updated values to the \c model properties. The QML ListModel is editable by default whereas C++ models must implement setData() to become editable. Integer and JavaScript array models are read-only. Supposed a \l{QAbstractItemModel} based C++ model that implements the \l{QAbstractItemModel::}{setData} method is registered as a QML type named \c EditableModel. Data could then be written to the model like this: \qml ListView { anchors.fill: parent model: EditableModel {} delegate: TextEdit { required property var model width: ListView.view.width height: 30 text: model.edit Keys.onReturnPressed: model.edit = text } } \endqml \note The \c edit role is equal to \l Qt::EditRole. See \l{QAbstractItemModel::}{roleNames}() for the built-in role names. However, real life models would usually register custom roles. \note If a model role is bound to a \l{Required Properties}{required property}, assigning to that property will not modify the model. It will instead break the binding to the model (just like assigning to any other property breaks existing bindings). If you want to use required properties and change the model data, make model also a required property and assign to \e model.propertyName. For more information, visit the \l{qtquick-modelviewsdata-cppmodels.html#changing-model-data}{Using C++ Models with Qt Quick Views} article. \section1 Using Transitions Transitions can be used to animate items that are added to, moved within, or removed from a positioner. Transitions for adding items apply to items that are created as part of a positioner, as well as those that are reparented to become children of a positioner. Transitions for removing items apply to items within a positioner that are deleted, as well as those that are removed from a positioner and given new parents in a document. \note Changing the opacity of items to zero will not cause them to disappear from the positioner. They can be removed and re-added by changing the visible property. */